This invention relates to a transmission power control system for a mobile communication network of a cellular type.
For a large-capacity mobile communication network, such as an automobile telephone network, a service area is composed of a plurality of radio or wireless zones (herein called cells). In each of the cells, a base station comprises a transmitter and a receiver to establish a radio communication channel for carrying out a bidirectional communication with a mobile station comprising a transmitter and a receiver. Such a network is said to be of a cellular type.
The transmitters of the base station and the mobile station have a maximum transmission power capable of sufficiently providing a signal to noise ratio (hereafter called a CNR) and a carrier to interference power ratio (hereafter called a CIR) even when the mobile station is near a boundary between the cells. It consequently results that each transmitter uses the transmission power beyond necessity when the mobile station is present near the base station. In order to avoid such waste of the power and to suppress an average power consumption, use is made of a technique of controlling the transmission power of each transmitter to make the base station and the mobile station have a reception level kept constant. This technique is called transmission power control (or power control) and is used in the automobile telephone network.
A conventional transmission power control system of keeping a reception power level constant is disclosed in a report contributed by Teruya FUJII and Masayuki SAKAMOTO to Conference Record of 38th IEEE Vehicular Technology Conference, June, 1988, pages 668-672, under the title of "Reduction of Cochannel Interference in Cellular Systems by Intra-Zone Channel Reassignment and Adaptive Transmitter Power Control" and in another report contributed by A. N. Rosenberg to Conference Record of 35th IEEE Vehicular Technology Conference, Colorado, May 1985, pages 12-15, under the title of "Simulation of Power Control and Voice-Channel Selection in Cellular Systems".
In the conventional transmission power control system of keeping a reception power level constant, the transmission power is kept constant despite an increase in an interference level. As a result, a deterioration is liable to frequently occur in a signal quality.